A Study On Fabrication Of Highly Efficient Copper Indium Selenium Thin Film Solar Cells Via Doctor Blading

Copper indium sulfur and its derivative（Cu（In,Ga）（S,Se）₂,CIGS）semiconductor materials with chalcopyrite structure have the advantages of high absorption coefficient,good stability and strong anti-radiation ability and adjustable band gap.It is an ideal light-absorbing material for thin film solar cells.At present,the highest efficiency of CIGS solar cells has reached 23.4%.However,commercial CIGS solar cells are all made by vacuum methods,which is costly and complicated,making it difficult to achieve large-scale production.The precursor solution method has the advantages of low cost,simple process and high material utilization rate.It is an ideal scheme for large-scale industrial production.However,there are still some problems to be studied in the application of fabricating solar cells with precursor solution:（1）The influence of heterojunction annealing treatment on device performance is unknown;（2）The spinning coating method is not suitable for the fabrication of absorber layers with large area and its material utilization rate is low;（3）The efficiency of the device prepared by solution method is relatively low compared with that from vacuum method.In order to solve the problems above,this thesis carries out the following aspects of work:（1）The influence of heterojunction heat treatment on the performance of CISSe devices.The heat treatment of copper indium selenide sulfide/cadmium sulfide（CISSe/Cd S）heterojunction at different temperatures showed that the heat treatment of the heterojunction decreased device performance,which is more serious with the increased treatment temperature.Raman test was performed on the devices before and after the heterojunction heat treatment,and it was found that the crystallization of the devices did not change after the heterojunction heat treatment.Furthermore,E_U numerical fitting,DLCP test and variable temperature I-V test were carried out on the devices before and after the heterojunction heat treatment,and it was concluded that the main reason for the degradation of device performance after the heterojunction heat treatment was the increase of the interface recombination of the heterojunction.（2）Fabrication of high efficiency CISSe thin film solar cells by optimizing the basic conditions and the numbers of layers of doctor-blading.In the process of coating CIS precursor films by doctor blade,it was found that the optimal dose of single solvent was 11μL.Under this condition,the substrate could be better paved and the precursor films with the largest effective area could be obtained.The optimal annealing temperature of precursor wet film is 110℃,under which most solvents can be volatilized and the decomposition of organic matter will not be affected.The optimal concentration of precursor solution was 0.5 mol/L and a clear and stable precursor solution could together with a film with good morphology of absorption layer could be prepared if it was employed.The optimum number of coating layers is 12,and under this condition,the best crystal,grain density and the largest size of the absorption layer film can be obtained.Using the optimized conditions above,the champion CISSe solar cell devices with an efficiency of 11.8%were fabricated by doctor-blading and its V_OC,J_SC and FF were 0.48 V,36.1 m A/cm² and 68.3%,respectively.（3）Improving film morphology and solar cell performance by K and Rb doping.By analyzing the J-V and EQE curves of devices with different K doping concentrations（0%,1%,2%,3%）,2%was found to be the best doping concentration.Compared with devices that are not doped,the V_OC,FF and PCE of the device are increased by 7%,13.5%,30%,respectively,and PCE of the champion device is 12.3%.By comparing the SEM cross-section and surface images of the absorbor layers,it can be deduced that the device performance is improved by the assisted grain growth of K₂Se in the liquid phase,which leads to larger and denser copper indium selenium crystals.Analyzing the extracted E_U value,a conclusion is drawn that the E_U value decreases from 17.6 me V to 14.9 me V after doping with K.Therefore,the doping of K effectively inhibits the band trailing phenomenon.By fitting the ideal factor n,we can infer that the defects of the device are effectively passivated when K doping concentration is 2%.By studying J-V and EQE curves of different Rb doping concentrations（0%,1%,2%,3%）,it is concluded that 1%Rb doping individually increases V_OC by 5.8%,FF by 5.4%,and PCE by 5.5%.By comparing SEM cross-section and surface views of absorber layers,it is found that Rb doping promotes the grain growth and increases grain size and from the extracted E_U value,a deduction that doping Rb element also inhibits the band trailing phenomenon is drawn.By comparing the performance parameters of doped K and Rb devices,it is found that K element improves the performance of devices more.